This invention relates to brushless DC motors with improved starting and more particularly to a brushless DC motor with an annular permanent magnet, stator windings at one location along the outer circumference of the magnet, and two dissimilar pole shoes or pole pieces to assure starting without cogging.
U.S. patent applications Ser. No. 416,504 of Deavers and Reffelt, filed Sept. 10, 1982 and Ser. No. 520,154 of Brown and Grouse, filed Aug. 4, 1983, both assigned to the assignee of this invention and incorporated herein by reference, relate to a simple brushless DC motor particularly useful as an inexpensive fan motor. A relatively large annular rotor permanent magnet magnetized in segments is driven by stator provisions located outside the annular magnet at a location on its circumference. In fan applications, the fan blades are located radially inwardly of the annular magnet.
In the motor of the first application mentioned above, the annular magnet has radially magnetized segments spaced apart by unmagnetized segments. A winding on a bobbin has a pair of pole pieces connected in flux conducting relation to it, the pole pieces extending from the ends of the bobbin to correctly placed locations near the outer surface of the annular magnet to attract the magnetic segments of the magnet with each energization of the winding. Commutation is by a Hall switch located near the outer circumference of the annular magnet and responsive to the magnetized segments to energize the winding at appropriate points in the travel of the annular rotor magnet. The winding is left unenergized during portions of the travel of the magnet. When the motor is turned off, there is, absent starting provisions, a likelihood that the magnet will come to rest in a position, in which, upon startup, the Hall switch will not energize the winding and no rotary movement will result. To overcome this, a rather large permanent detent magnet segment is located to attract a permanent magnet segment and assure that the annular rotor magnet comes to rest correctly with each shutdown of the motor. However, the force of this detent magnet lying just outside the circumference of the annular magnet contributes to vibration of the motor.
In the second of the above-mentioned U.S. patent applications, a motor with a similar layout employs an annular rotor magnet permanently magnetized at alternate segments oppositely radially magnetized. A pair of coils are wound onto the bobbin in bifilar fashion and are alternately energized to create, via pole pieces proximate the annular rotor magnet, a reversing electromagnetic field. Consequently there are no rather large dead spots in the operation of the motor, but still the possibility of cogging exists, just as in any permanent magnet DC motor. Experimentally, a piece of unmagnetized magnetic material located near the outer circumference of the annular magnet was positioned to attract the nearest permanent magnet segment and draw the rotor to rest at a position angularly offset from a cog position in the event that the rotor was about to come to rest at the cog position. Location of the piece of magnetic material, typically a steel pin, proved sufficient to assure start-up in experimental units. In production, however, cogging proved more prevalent than expected with this arrangement. Hence, it became necessary to develop an anti-cogging arrangement to assure that the motor would consistently start even when manufactured under ordinary production conditions.
Anti-cogging arrangements are known in the art, these include modification of pole faces to bring one location of a stator pole face closer a rotor magnet across the air gap than another location on the same pole face. In motors like those to which this invention applies, modification of one pole piece to reduce its area opposite the outer surface of the annular magnet was tried, for example by shortening the end of one pole piece where it was closely proximate the rotor magnet. Unsatisfactory results were achieved even when it was expected that such modification should result in consistent starting. Only a very radical modification of one pole resulted in consistent starting of production motors.